1. Field of the Invention
The present invention relates to a plasma processing apparatus for performing, for example, film formation, etching, or ashing by using a plasma.
2. Description of the Related Art
In the semiconductor fabrication processes, processes using a plasma such as film formation, etching, and ashing are performed. In these processes, various processing gases for film formation or film removal are used. These processing gases are formed into plasma by appropriate plasma generating means in a processing vessel and react with the surface of a semiconductor wafer in accordance with the respective purposes, thereby performing film formation or film removal. The waste gases produced by the process are exhausted from the processing vessel by an exhaust device such as a vacuum pump.
Various plasma generating means are used in these plasma processes. Among various plasma generating means a plasma processing apparatus including parallel plate electrodes is being extensively used because the apparatus can be a uniform process, can process large-diameter semiconductor wafers, and has a relatively simple construction.
Generally, the parallel-plate plasma processing apparatus comprises a lower electrode for holding a semiconductor wafer in a processing vessel, and an upper electrode arranged above and parallel to the lower electrode with a predetermined spacing between them. For example, while a reactive gas is supplied from a large number of holes formed in the surface of the upper electrode which opposes the lower electrode, a high-frequency power is applied to the lower electrode to generate a plasma between the two electrodes. This plasma is used to perform an etching or the like process for a semiconductor wafer.
To etch a silicon oxide film or a silicon nitride film formed on the surface of a semiconductor wafer, a CF-based gas such as CF.sub.4 or CHF.sub.3 is conventionally used as a processing gas.
In the case of etching using a CF-based gas, for example, this CF-based gas is formed into a plasma to produce active species such as ions or radicals. These active species react with a substance such as a silicon oxide film exposed into a predetermined pattern, thereby producing volatile gases such as SiF.sub.4 and CO.sub.2. These volatile gases are exhausted and removed from the processing vessel to the outside.
If the processing gas completely, stoichiometrically reacts with the substance on the wafer surface during this plasma process such as etching, no unreacted substance is produced. Therefore, after the reaction product is exhausted outside the processing vessel as described above, no residual substance exists in the processing vessel. However, a part of the processing gas is exhausted as it is left unreacted. Also, some active species recombine without reacting with the substance, producing a fluorocarbon-based polymer, such as C.sub.X F.sub.Y or C.sub.X F.sub.Y O.sub.Z, as a nonvolatile reaction product. This nonvolatile reaction product adheres and deposits on the inner surface of the reaction vessel, and the deposit thus formed forms particles.